The present invention relates to an interior member or indoor equipment using a titanium oxide particle as a photocatalyst. More specifically, the present invention enables an organic material or nitrogen oxide adhering to the surface of an interior member or indoor equipment to be decomposed by the irradiation of a weak ultraviolet ray.
Photocatalysis is a reaction of decomposing an organic material or nitrogen oxide present in air or water using light energy in the presence of a photocatalyst. Thus, photocatalysis is a technique which uses an ultraviolet ray present in the light emitted from sunlight or an interior room illuminator such as a fluorescent lamp and an incandescent lamp, and studies thereon are being made broadly, for example, on the application to architectural materials or the cleaning of water or air.
As the photocatalyst, zinc oxide, cadmium selenide, gallium arsenide and the like are known but titanium dioxide (hereinafter referred to as xe2x80x9ctitanium oxidexe2x80x9d) is usually used. Titanium oxide is known to assume three kinds of crystal structures, and in any structure, a distorted octahedron formed by the 6-coordination of oxygen atoms to titanium atom is fundamental. The three kinds of crystal structures are a tetragonal anatase type for low temperature, a rutile type for high temperature and a rhombic brookite type for a temperature between those two types. Among these, anatase titanium oxide is considered to be most excellent as a photocatalyst and a large number of studies have heretofore been made thereon.
It is known that the decomposing action of titanium oxide on contamination caused by adhesion of an organic material or on the generation of mold is generally accelerated by the action of an ultraviolet ray present in sunlight (see, Unexamined Re-Published International Patent Application WO96-29375). The mechanism in the decomposing action of titanium oxide is not yet fully understood, however, the decomposition presently is considered to proceed as follows.
When titanium oxide absorbs an ultraviolet ray, two kinds of electric charges (carriers) of an electron and a hole are generated inside and the hole is induced on the surface thereof. This hole acts with water on the particle surface to generate an OH radical having a strong oxidizing power on the surface of the particle and the pollutant positioned on the particle surface, such as nitrogen oxide, is decomposed by the OH radical into carbon dioxide gas, water or a low molecular weight material. These decomposition products accumulate on the particle surface and are washed out by rain or the like. As a result, the photocatalytic action of the particle is regenerated. Thus, the photocatalytic function of titanium oxide can be continuously maintained. (See, Akira Fukushima, Kazuhito Hashimoto and Toshiya Watanabe, Titanium Oxide Photocatalysis Fundamentals and Applications, BKC Inc., (1998)).
The present inventors have found that titanium oxide particles containing a brookite-type crystal exhibit a strong photocatalytic effect even under a weak ultraviolet ray. The present invention has been accomplished based on this finding.
An anatase titanium oxide as a photocatalyst has been mainly used. Irradiation energy of at least 0.005 mW/cm2 at a wavelength of 365 nm is needed to exhibit the function of a photocatalyst. Therefore anatase titanium oxide as a photocatalyst has been only adopted on sunshiny areas such as the windows side of an indoor place or in an outdoor place. Brookite titanium oxide used in the present invention exhibits its photocatalytic function under the condition of a weak ultraviolet ray having an irradiation energy of, for example, 0.001 mW/cm2 and under the conditions of light in indoor places.
The present inventors have thought that since the photocatalytic property of a titanium oxide particle is influenced by the OH radical concentration on the surface thereof or the like, selection of the crystal structure, surface shape or particle size of titanium oxide and control of the shape are matters of importance.
In other words, the generation of OH radicals is governed by the generation density of holes and the life thereof in the above-described mechanism and the generation density and life of a hole is governed by the band gap thereof. Therefore, the photo-functional property of titanium oxide is considered to be dependent on the crystal system. It is well known that a rutile-type crystal has a band gap of 3.0 eV and an anatase-type crystal has a band gap of 3.2 eV.
Furthermore, the OH radical is generated by the reaction of a hole with water, accordingly, the hole potential (corresponding to the potential of an electric charge depletion layer in the valence band) on the surface of titanium oxide plays an important factor. This potential is governed by the band system in the crystal structure, therefore, also in this point, the photo-functional property of titanium oxide is inferred to be dependent on the crystal system.
However, in the case of decomposing organic material or the like adsorbed, for example, on an interior member by letting titanium oxide be present on the surface of an interior member or indoor equipment in a room, a tunnel, a car, a plane or a ship, the ultraviolet ray which can be used is a weak ray emitted from an interior room illuminator and the like. Accordingly, unless a specific ultraviolet ray source such as a strong ultraviolet ray lamp is installed, sufficiently high percentage decomposition cannot be attained.
The present invention has been made under these circumstances and a primary object of the present invention is to obtain an interior member or indoor equipment capable of decomposing an organic material or nitrogen oxide adsorbed on wall paper, floor member or gauges, with the irradiation of a weak ultraviolet ray.
Another object of the present invention is to provide an interior member or indoor equipment where titanium oxide particles containing a brookite-type crystal are present on the surface thereon.
Still another object of the present invention is to provide a method of using titanium oxide particles containing a brookite-type crystal under the condition of irradiation with an ultraviolet ray having irradiation energy of from 0.001 to 0.2 mW/cm2 at a wavelength of 365 nm.
The titanium oxide particles for use in the present invention contain brookite-type crystal. The titanium oxide particle is not particularly limited as long as brookite titanium oxide particle is present. The brookite titanium oxide particle may be used alone or a rutile or anatase titanium oxide particle may also be present. In the case of a rutile or anatase titanium oxide particle is present, the ratio of the brookite titanium oxide particle in the titanium oxide particles is not particularly limited but it is usually from 1 to 100 wt %, preferably from 10 to 100 wt %, more preferably from 50 to 100 wt %. This is because the brookite titanium oxide is superior in photocatalytic activity to rutile or anatase titanium oxide.
The titanium oxide particles containing a brookite-type crystal may be produced by a vapor-phase production process where anatase titanium oxide particles are heat-treated to obtain titanium oxide particles containing a brookite-type crystal, or by a liquid-phase production process where a solution of a titanium compound such as titanium tetrachloride, titanium trichloride, titanium alkoxide or titanium sulfate is neutralized or hydrolyzed to obtain a titanium oxide sol having dispersed therein titanium oxide particles.
The production process is not particularly limited as far as titanium oxide particles containing a brookite-type crystal can be obtained. However, taking account of the photocatalytic activity and manageability of the material obtained and in the case of forming a titanium oxide thin film, transparency and adhesion, a production process previously found by the present inventors (Japanese Patent Application Nos. 9-231172 and 10-132195) is preferred, where titanium tetrachloride is added to hot water at from 75 to 100xc2x0 C. and then hydrolyzed at a temperature of from 75xc2x0 C. to the boiling point of the solution while controlling the chloride ion concentration. Thereby titanium oxide particles containing brookite-type crystal are obtained as a titanium oxide sol.
The titanium oxide particles containing brookite-type crystal are not particularly limited in terms of the size thereof but usually have an average particle size of from 0.005 to 0.1 xcexcm. If the average particle size exceeds 0.1 xcexcm, the titanium oxide particles are reduced not only in photocatalytic activity but also in transparency. Therefore, when such titanium oxide particles are present on the surface of an interior member or the like, the color of the titanium oxide adversely affects the color of the interior member or the like. If the average particle size is less than 0.005 xcexcm, the titanium oxide particle is difficult to handle during production thereof. The titanium oxide particle usually has a specific surface area of 20 m2/g or more.
The thus-obtained titanium oxide particles containing a brookite-type crystal are allowed to be present on the surface of an interior member or indoor equipment by a method of coating a sol or solution of titanium oxide particles containing brookite-type crystal on an interior member substrate and thereafter drying or heat treating it to bond the particles to the surface of the interior member substrate or the like, or by a method of mixing titanium oxide particles containing brookite-type crystal with a coating material or the like and coating it on an interior member substrate, followed by drying or heat treatment. The titanium oxide particles containing brookite-type crystal may also be fixed on the interior member substrate or the like using, for example, an adhesive.
The method of coating a sol or solution of titanium oxide particles containing a brookite-type crystal on an interior member substrate is not particularly limited and any known method such as spin coating, flow coating, dip coating, spray coating, bar coating, roller coating or brush coating may be used. In the case of coating a titanium oxide sol, the amount coated is usually from 0.01 to 0.2 mm in terms of the thickness of the film coated.
In the case where a sol or solution of titanium oxide particles containing brookite-type crystal is coated on an interior substrate and thereafter dried or heat treated to bond the particles to the surface of the interior member substrate or the like, an adhesive may be added to the titanium oxide sol or solution. The adhesive is used to increase the adhesion strength between the titanium oxide particle and the interior member substrate. Although the adhesive is not particularly limited so far as it has a photocatalysis resistance, an oxide such as silicon oxide, aluminum oxide, zirconium oxide, calcium oxide and magnesium oxide, or an alkoxide as a precursor of these oxides is usually used.
These adhesives may be used individually or as a combination of two or more thereof. In the case of using a combination of adhesives, the mixing ratio may be freely selected. The amount of the adhesive added is, when the adhesive is calculated as an oxide, usually from 10 to 80 parts by weight per 100 parts by weight of titanium oxide particles. If the amount used exceeds 80 parts by weight, the ratio of titanium oxide particles buried in the adhesive increases and the photocatalytic activity decreases, whereas if it is less than 10 parts by weight, the effect ascribable to the adhesive addition may not be obtained.
The method for adding the adhesive is not particularly limited but a method of adding an adhesive to a titanium oxide sol or solution and then coating it on an interior substrate or a method of coating a titanium oxide sol or solution by spraying and at the same time, coating an adhesive by another spraying may be used.
In the case where a sol or solution of titanium oxide particles containing brookite-type crystal is coated on an interior material substrate and then dried, an appropriate solvent may be added so as to increase the drying rate. When the titanium oxide sol is dispersed in water, an organic solvent such as ethyl alcohol is usually used.
Before coating a sol or solution of titanium oxide particles containing brookite-type crystal on an interior material substrate, a solution containing a silica or fluorine resin may be coated on the interior member substrate and then dried to form a protective film on the interior member or the like.
After the sol or solution of titanium oxide particles containing brookite-type crystal is coated as above, the sol or solution is dried or heat-treated so as to fix the titanium oxide particles on the interior member substrate. The atmosphere for the drying or heat treatment is not particularly limited. The drying or heat treatment may be performed in atmospheric air, under vacuum or in an inert gas but it is usually performed in atmospheric air. The drying or heat treatment temperature is usually from 20 to 800xc2x0 C., preferably from 20 to 150xc2x0 C., and the drying or heat treatment time is usually from 5 minutes to 24 hours, preferably from 15 minutes to 12 hours.
In the case of using an interior member substrate with heat resistance, such as metal, ceramics or glass, the titanium oxide particle may be calcined after the drying or heat treatment. The calcining is performed to more firmly bond the titanium oxide particle to the interior member substrate or the like and in the case of a titanium oxide particle thin film, the calcining is performed to improve the hardness. The atmosphere for the calcining is not particularly limited and the calcining may be performed in atmospheric air, under vacuum or in an inert gas but is usually performed in atmospheric air. The calcining temperature varies depending on the kind of interior member substrate or the like, however, it is usually from 200 to 800xc2x0 C. If the calcining temperature is less than 200xc2x0 C., the effect of improving the adhesion of titanium oxide particles or the hardness of titanium oxide thin film may not be attained by the calcining, whereas if it exceeds 800xc2x0 C., the brookite-type crystal transfers to the rutile-type crystal and the photocatalytic performance is disadvantageously reduced. The burning time is not particularly limited but is usually from 1 to 60 minutes.
In the thus-obtained interior member or indoor equipment where titanium oxide particles containing brookite-type crystal are present on the surface thereof, when the titanium oxide particles are present as a thin film, the thickness of the film is usually from 0.05 to 2 xcexcm. If the film thickness is less than 0.05 xcexcm, a sufficiently high photocatalytic performance may not be attained, whereas if it exceeds 2 xcexcm, the photocatalysis takes place only in the vicinity of the surface of the titanium oxide thin film, as a result, titanium oxide not participating in the photocatalysis increases disadvantageously in view of profitability and the thin film is readily stripped off from the interior member substrate or the like.
The titanium oxide particles present on the interior member substrate may not be a thin film but may be, for example, in the form of islands.
The interior member or indoor equipment with titanium oxide particles containing a brookite-type crystal present on the surface thereof of the present invention can decompose an organic material or nitrogen oxide in air, adsorbed to the interior member or the like by photocatalysis of the titanium oxide particle. The interior member is not particularly limited as long as it is used indoors or in a car, a plane, a ship or a tunnel. Examples of interior members for indoor use such as use in a room, a bath or a kitchen, include wall paper, floors, wall tiles, window glass, curtains, blinds, luminaires, air-cooling and heating appliances such as air conditioners and stoves, OA instruments such as personal computers, AV instruments such as television and stereo, telephones, clocks, beds, lockers, cages, tableware, ashtrays, sinks, cooking ranges, fans and hoods thereof, dining tables or chairs, desks, household Buddhist shrines, family altars, cisterns such as goldfish basin, dressers, bathtubs, wash stands and stools. Examples of the interior member for use in laboratories and the like include draft chambers and glove boxes. Examples of interior member for use in a car, a plane or a ship include interior members such as seats, windshield glass, steering wheels, cover glasses of gauges, and radiation fins of a regenerator. Examples of interior member for use in a tunnel include inner walls, cover glass of illuminators, indicators, guide signs and a signboards.
The interior member or indoor equipment of the present invention is characterized by the use of titanium oxide particles containing brookite-type crystal, so that this interior member or the like reveals excellent photocatalysis even under irradiation with weak ultraviolet ray having an irradiation energy of from 0.01 to 0.2 mW/cm2 at a wavelength of 365 nm as compared with interior members with titanium oxide particles of anatase or rutile-type crystal present on the surface thereof.
The interior member or indoor equipment thus having titanium oxide particles containing brookite-type crystal present on the surface thereof of the present invention enables photocatalysis to proceed efficiently even with a weak ultraviolet ray present in the interior room illuminator generally present in the living space, exhibits an excellent percentage decomposition for organic materials or nitrogen oxide adsorbed on an interior member or the like, and is suitable for use in a room, a car, a plane, a ship or a tunnel.